I bought a '97 E420 with 98K miles, and (wouldn't you know it) the next day the Check Engine Light came on. I took the car to a shop that works on MB, and I was told that the scanner indicated both oxygen sensors on the left side needed to be replaced. However, when the codes were deleted, the CEL didn't come back on --- at least not until the next day.

I contacted the used car dealer who sold me the car, and he said the left front oxygen sensor had been replaced about 4k miles earlier. He said it was the correct Bosch OE plug-in model, not a splice-in model. Before I bought the car he had shown me recent smog test results which showed the car was very clean. Also, the CARFAX on the car looked really good, and it had never failed a smog test. The car starts and runs fine.

The dealer has a good reputation, and I think he's telling me the truth. I suppose it is possible the new left front sensor has gone bad in such a short period, but that doesn't seem likely. I don't have any maintenance records, so I don't know when the plugs have been replaced or if there have been any problems with the fuel injectors, etc.

I'm about to take it back to the shop, and I'll bet they want to replace both oxygen sensors on the left side. Any suggestions about what I might ask them to do before they do that? Any ideas about what might be causing the CEL other than the oxygen sensors (assuming the shop reads the codes right)? Would appreciate any ideas. Thanks.

 

I would have replaced the the pre- and post-cat O2 sensors as a set the first time.

I'm not sure if it was a question or not, but it takes a few miles for some (not all) of the codes to set and turn on the CEL. Actually, it's not the number of miles but the number of times a pass/fail test either passes or fails. It's not a knee-jerk reaction system, if you will. Tests continually fail, but the ECU immediately corrects the cause of the failure by making an adjustment, such as the air/fuel mix or ignition timing. When it can't pass a test by making a correction that satisfies predetermined emission parameters, it lights the CEL.

Also, it's not unusual for a car to be able to pass emissions testing with the CEL turned on. A well-maintained engine is far cleaner than most states' requirements for emissions, and is still cleaner even though an emissions-related part may be defective. However, in my state (GA) the mere fact that the CEL is turned on would result in a failed test if the car is a '96 or newer--that is, it has OBDII.

 

Thanks, Lugnut, for your input.

I've driven the car about 275 miles in the two weeks since I bought it, and that has been a combination of 60-75 mph driving for up to 80 miles and some stop-and-go city driving. If I understand what you're saying, it seems that many miles should be enough for the ECU to figure out that it can't correct the situation and call for help. Do you think that's right?

Also, if it's something as simple as a fouled spark plug, will the scanner still show the code for oxygen sensor, or will it show another code? I don't have any maintenance history on the car, so I don't know what has been done to it.

I was reading the tech notes from one parts house on the net, and they recommended changing out all oxygen sensors at 100k miles - - of course, they're in the business of selling them. What do you think about putting in all new ones?

Thanks again for your help.

 

You don't absolutely have to change all the sensors. If it were me, I would. That's probably overkill, but that's the way I am. I wouldn't want to have to change another sensor in 3 months. It's possible for bad gas or some off-the-shelf additives or chemicals to damage O2 sensors. They are very sensitive to certain chemicals. If that's what caused the failure, you can expect the other sensors to go bad.

A fouled plug wouldn't cause an O2 sensor fault code. But misfires are tracked on OBDII cars. I don't know the specifics on how, but it can turn the CEL on.

I suspect the dealer disconnected the battery before you bought it. He may or may not have done it to deliberately clear the code and turn the CEL off to deceive you. There are other legitimate reasons to disconnect the battery.

The ECU has tables in matrices that need to be filled with data. As you drive, data is gathered and stored in the tables. Samples are taken from many driving conditions as the ECU "learns" of the engine's condition. For example, idling in gear, idling in park, driving at specific RPMs, coasting to a stop, AC on and off, etc, etc, etc. In your case, you drove 275 miles before the specific condition arose which the ECU could not correct. My Corvette has OBDII. I can track and fill all the tables with data in under 20 miles.

 

Since my posts in March, I've continued to have problems with the CE light. I had all 4 oxygen sensors replaced with new OE Bosch sensors, and I had new Bosch OE spark plugs put in. This work was done by a shop I've been taking my '73 280 to for many years ...... they work on MBs of all vintages.

After the new sensors were put in, the CE light came on in about 30 miles. Shop scanned it and said oxygen sensors (not sure of the code number). They then worked on the connector to the computer (said they forced all the female connectors together with needle nose pliers to get a tighter connection). CE light came on in about 220 miles. Shop scanned it and got codes PO 140 and PO 133, which they tell me are oxygen sensor codes .... not sure which bank.

The shop owner is now at his wits end and recommends I take the car to a dealer. Any suggestions what else I might try before I get ready for an out-of-sight bill? If you have had a similar problem and live in S. California, please let me know who solved it for you. Many thanks for your assistance.